It's "aspect ratio", not aspect "ration".
It's what determines whether you have square pixels ("normal") or, if rectangular pixels, what the aspect ratio (width : length) of that rectangle is.
It's explained in the Help files. I cannot go into more detail because you have neglected to provide information about your platform and exact version of Photoshop.
Example in next post
Thank you so much for your answer to my question about pixel aspect
ratio. It's very clear, and I really appreciate your help.
You're welcome, Stephanie.